I say cyberdruid is right (though i dont have any links to back it up atm) because I ran vista on a desktop with 1gb of ram. Idle was ~200mb of ram at any given moment, and vista ran just fine. Then I had a toshiba laptop with 2gb of ram. Idle could get up to 512 or even more mb of ram, vista still ran fine. Now on my my asus laptop with 4gb of ram it can get up to 2 gb at idle! However, even crysis at high doesnt stutter, and on my little lcd screen that shows ram usage it never gets even close to 80% used so I assume vista throttles back the prefetch in the ram and whatnot so that it can be used for currently running programs. Also, vista itself is silky smooth, and programs open super fast. So im assuming like cyberdruid says, the prefetch uses up as much ram as it can justify so your programs can open faster, then throttles it back when you game or multitask heavily.
Plus Im pretty sure it says all this in the bragging section of the microsoft website for vista.